Using Flexible NetFlow Flow Sampling

This document contains information about and instructions for configuring sampling to reduce the CPU overhead of analyzing
traffic with Flexible NetFlow.

NetFlow is a Cisco technology that provides statistics on packets flowing through a router. NetFlow is the standard for acquiring
IP operational data from IP networks. NetFlow provides data to support network and security monitoring, network planning,
traffic analysis, and IP accounting.

Flexible NetFlow improves on original NetFlow by adding the capability to customize the traffic analysis parameters for your
specific requirements. Flexible NetFlow facilitates the creation of more complex configurations for traffic analysis and data
export through the use of reusable configuration components.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information,
see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module,
and to see a list of the releases in which each feature is supported, see the feature information table.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature
Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Prerequisites for Using Flexible NetFlow Flow Sampling

The networking device must be running a Cisco release that supports Flexible NetFlow.

Restrictions for Using Flexible NetFlow Flow Sampling

Information About Flexible NetFlow Flow Sampling

Flow Samplers

Flow samplers are
created as separate components in a router’s configuration. Flow samplers are
used to reduce the load on the device that is running
by limiting the number of packets that
are selected for analysis.

Flow sampling
exchanges monitoring accuracy for router performance. When you apply a sampler
to a flow monitor, the overhead load on the router of running the flow monitor
is reduced because the number of packets that the flow monitor must analyze is
reduced. The reduction in the number of packets that are analyzed by the flow
monitor causes a corresponding reduction in the accuracy of the information
stored in the flow monitor’s cache.

Samplers are combined
with flow monitors when they are applied to an interface with the
ip flow monitor command.

How to Configure Flexible NetFlow Flow Sampling

Flow sampling reduces the CPU overhead of analyzing traffic with Flexible NetFlow by reducing the number of packets that
are analyzed.

Note

Only the keywords and arguments required for the Flexible NetFlow commands used in these tasks are explained in these tasks.
For information about the other keywords and arguments available for these Flexible NetFlow commands, refer to the
Cisco IOS Flexible NetFlow Command Reference .

Configuring a Flow Monitor

Samplers are applied to an interface in conjunction with a flow monitor. You must create a flow monitor to configure the
types of traffic that you want to analyze before you can enable sampling. Perform this required task to configure a flow monitor.

Each flow monitor has a separate cache assigned to it. Each flow monitor requires a record to define the contents and layout
of its cache entries. The record format can be one of the predefined record formats, or an advanced user may create his or
her own record format using the
collect and
match commands in Flexible NetFlow flow record configuration mode.

Note

You must use the
no ip flow monitor command to remove a flow monitor from all of the interfaces to which you have applied it before you can modify the parameters
for the
record command on the flow monitor.

Example:

Configuring and Enabling
Flow Sampling

Perform this
required task to configure and enable a flow sampler.

Note

When you specify
the "NetFlow original," or the "NetFlow IPv4 original input," or the "NetFlow
IPv6 original input" predefined record for the flow monitor to emulate original
NetFlow, the flow monitor can be used only for analyzing input (ingress)
traffic.

When you specify
the "NetFlow IPv4 original output" or the "NetFlow IPv6 original output"
predefined record for the flow monitor to emulate the Egress NetFlow Accounting
feature, the flow monitor can be used only for analyzing output (egress)
traffic.

MIBs

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use
these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products
and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

Feature Information for Flexible NetFlow Flow Sampling

The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1. Feature Information for Flexible Netflow Flow Sampling

Feature Name

Releases

Feature Information

Flexible Netflow - Random Sampling

12.2(50)SY

12.4(20)T

Cisco IOS XE Release 3.1S

Cisco IOS XE Release 3.2SE

Flow samplers are created as separate components in a router’s configuration. Flow samplers are used to reduce the load on
the device that is running Flexible NetFlow by limiting the number of packets that are selected for analysis. Samplers use
either random or deterministic sampling techniques (modes).